1. Field of the Invention
The present invention relates to gate valve structures used primarily in large mains carrying fluid under high pressure such as water mains in city and country water distribution systems. It will be apparent, however, to those skilled in this art, that while the design considerations giving rise to the present invention are directly primarily to problems resulting from high pressure water distribution systems, the valve of the present invention can be successfully utilized in a wide range of water main sizes and in other fluid distribution systems such as fire protection and waste water systems as well as chemical, steam, gas, oil or the like systems, wherein the fluid is under varying pressures from low to high pressure and may contain corrosive chemicals.
2. Description of the Prior Art
The prior art has proposed a wide variety of structures for gate valves used in water distribution systems for the purpose of expanding the utility of the valve structure as well as prolonging its useful life before replacement or significant repair is required. To this end, structures such as are disclosed in U.S. Pat. No. 3,662,778, granted May 16, 1972, have been proposed where a portion of the valve plate and the guides in the housing for the valve plate have been coated to reduce wear on the housing and valve plate. Similarly, in prior U.S. Pat. Nos. 3,763,880 of Oct. 9, 1973 and 3,633,873 of Jan. 11, 1972, fully coated valve members have been proposed of specific structure to reduce the corrosive influence of the fluids being handled on the valve plate and its sealing surfaces. Another problem that the prior art has attempted to solve is that of the physical integrity of the housing for the gate valve since premature leakage of the housing can result in corresponding premature failure of the valve function itself. To this end, the elimination of securing members such as bolts, rivets and the like has been attempted, such as represented by U.S. Pat. No. 4,431,163 of Feb. 14, 1984.